Cut Down rework rune idea based on Blade of the Ruined King
Cut Down You deal 2% enemy champion's current health as bonus adaptive damage with abilities and basic attacks. If the target has more max health than you then you also gain 2 adaptive force (AD/AP) per 100 max health difference.
The idea is to turn Cut Down into mini Blade of the Ruined King which will be very effective against enemies with higher amount of health but the lower current health they have the less effective the rune becomes. Blade of the ruined king does it's job very well against champions with lots of health and not that well against glass cannons. So it makes sense that we turn Cut Down to act in a similar way to the item itself
Coup the Grace will always be effective because every champion can go down below 40% health. However current Cut down is not effective even against tanks or at least I personally never felt I am doing something against them
Nearly no one takes Cut Down because it's very uneffective. Not to mention that some champions have higher base health than champions that suppose to be tankier. Also % damage is far more effective against class cannons than tougher champions, who got health, armor and magic resistance mix. We used to have item called Giant Slayer that does the same thing as Cut down so the rune also have the same issue. However BORK does it's job well so turning Cut down into mini BORK would make the rune more desireable and more successful way to deal with tanks.